I'm good friends with an ex. She's a great person, but we just don't work romantically. For two years, I've been seeing a woman I love and want a future with. She initially said she was fine with my friendship with my ex. Two months ago, she said she was uncomfortable with it and it might even be a deal breaker. How is it fair for her to decide this now?

--Don't Wanna Dump A Friend

There are a number of things absent from straight men's friendships with other men -- namely how two dudes boozing it up together on the couch never leads to anyone's bra being yanked off and flung onto the ceiling fan.

Two years ago, your girlfriend did say she was okay with your friendship with your ex. So, your feeling like you've been played is understandable -- but probably driven the (very common!) tendency to overestimate our ability to engage in reliable "affective forecasting." "Affect" is researcher-ese for emotion, and affective forecasting involves predicting how some future event will make us feel. Research by psychologists Daniel Gilbert and Timothy Wilson suggests we're pretty bad at foreseeing what we'll ultimately want and how happy or unhappy it will make us down the road.

Our guesses about how we'll eventually feel are colored by our circumstances and preferences at the time we're making a prediction. For example, before your girlfriend was very attached to you, she might've believed your friendship with your ex was (and would keep being) no biggie. As her love for you grew, the stakes of losing you loomed large in a way they didn't back in the cool light of "Mmmkay, let's see where things go with Mr. (Possibly) Right."

Tell her you want to understand her feelings -- and do something few people do when they have a goal of their own in mind: Listen fully and open-mindedly (as opposed to giving the appearance of listening while mentally cataloging all the fantastic points you'll make). Hearing her fears could help you empathize with her -- which should make her feel understood. Explain why she has nothing to worry about (uh, assuming that's the case). You might also actively reassure her: regularly do stuff to show how much you love her. Ultimately, however, you might have a big ugly choice to make if you can't get your girlfriend to stop seeing your friendship with your ex as something along the lines of Wile E. Coyote getting the night watchman gig at KFC.

For pages and pages of "science-help" from me, buy my latest book, "Unf*ckology: A Field Guide to Living with Guts and Confidence." It lays out the PROCESS of transforming to live w/confidence.